Antibody-based therapeutics have been used successfully to treat a variety of diseases, including cancer and autoimmune/inflammatory disorders. Yet improvements to this class of drugs are still needed, particularly with respect to enhancing their clinical efficacy. One avenue being explored is the engineering of additional and novel antigen binding sites into antibody-based drugs such that a single immunoglobulin molecule co-engages two different antigens. Such non-native or alternate antibody formats that engage two different antigens are often referred to as bispecifics. Because the considerable diversity of the antibody variable region (Fv) makes it possible to produce an Fv that recognizes virtually any molecule, the typical approach to bispecific generation is the introduction of new variable regions into the antibody.
A number of alternate antibody formats have been explored for bispecific targeting (Chames & Baty, 2009, mAbs 1[6]:1-9; Holliger & Hudson, 2005, Nature Biotechnology 23[9]:1126-1136; Kontermann, mAbs 4(2):182 (2012), all of which are expressly incorporated herein by reference). Initially, bispecific antibodies were made by fusing two cell lines that each produced a single monoclonal antibody (Milstein et al., 1983, Nature 305:537-540). Although the resulting hybrid hybridoma or quadroma did produce bispecific antibodies, they were only a minor population, and extensive purification was required to isolate the desired antibody. An engineering solution to this was the use of antibody fragments to make bispecifics. Because such fragments lack the complex quaternary structure of a full length antibody, variable light and heavy chains can be linked in single genetic constructs. Antibody fragments of many different forms have been generated, including diabodies, single chain diabodies, tandem scFv's, and Fab2 bispecifics (Chames & Baty, 2009, mAbs 1[6]:1-9; Holliger & Hudson, 2005, Nature Biotechnology 23[9]:1126-1136; expressly incorporated herein by reference). While these formats can be expressed at high levels in bacteria and may have favorable penetration benefits due to their small size, they clear rapidly in vivo and can present manufacturing obstacles related to their production and stability. A principal cause of these drawbacks is that antibody fragments typically lack the constant region of the antibody with its associated functional properties, including larger size, high stability, and binding to various Fc receptors and ligands that maintain long half-life in serum (i.e. the neonatal Fc receptor FcRn) or serve as binding sites for purification (i.e. protein A and protein G).
More recent work has attempted to address the shortcomings of fragment-based bispecifics by engineering dual binding into full length antibody-like formats (Wu et al., 2007, Nature Biotechnology 25[11]:1290-1297; U.S. Ser. No. 12/477,711; Michaelson et al., 2009, mAbs 1[2]:128-141; PCT/US2008/074693; Zuo et al., 2000, Protein Engineering 13[5]:361-367; U.S. Ser. No. 09/865,198; Shen et al., 2006, J Biol Chem 281[16]:10706-10714; Lu et al., 2005, J Biol Chem 280[20]:19665-19672; PCT/US2005/025472; expressly incorporated herein by reference). These formats overcome some of the obstacles of the antibody fragment bispecifics, principally because they contain an Fc region. One significant drawback of these formats is that, because they build new antigen binding sites on top of the homodimeric constant chains, binding to the new antigen is always bivalent.
For many antigens that are attractive as co-targets in a therapeutic bispecific format, the desired binding is monovalent rather than bivalent. For many immune receptors, cellular activation is accomplished by cross-linking of a monovalent binding interaction. The mechanism of cross-linking is typically mediated by antibody/antigen immune complexes, or via effector cell to target cell engagement. For example, the low affinity Fc gamma receptors (FcγRs) such as FcγRIIa, FcγRIIb, and FcγRIIIa bind monovalently to the antibody Fc region. Monovalent binding does not activate cells expressing these FcγRs; however, upon immune complexation or cell-to-cell contact, receptors are cross-linked and clustered on the cell surface, leading to activation. For receptors responsible for mediating cellular killing, for example FcγRIIIa on natural killer (NK) cells, receptor cross-linking and cellular activation occurs when the effector cell engages the target cell in a highly avid format (Bowles & Weiner, 2005, J Immunol Methods 304:88-99, expressly incorporated by reference). Similarly, on B cells the inhibitory receptor FcγRIIb downregulates B cell activation only when it engages into an immune complex with the cell surface B-cell receptor (BCR), a mechanism that is mediated by immune complexation of soluble IgG's with the same antigen that is recognized by the BCR (Heyman 2003, Immunol Lett 88[2]:157-161; Smith and Clatworthy, 2010, Nature Reviews Immunology 10:328-343; expressly incorporated by reference). As another example, CD3 activation of T-cells occurs only when its associated T-cell receptor (TCR) engages antigen-loaded MHC on antigen presenting cells in a highly avid cell-to-cell synapse (Kuhns et al., 2006, Immunity 24:133-139). Indeed nonspecific bivalent cross-linking of CD3 using an anti-CD3 antibody elicits a cytokine storm and toxicity (Perruche et al., 2009, J Immunol 183[2]:953-61; Chatenoud & Bluestone, 2007, Nature Reviews Immunology 7:622-632; expressly incorporated by reference). Thus for practical clinical use, the preferred mode of CD3 co-engagement for redirected killing of targets cells is monovalent binding that results in activation only upon engagement with the co-engaged target.
Somatostatins are neuropeptides that act as endogenous inhibitory regulators. Somatostatins have a broad range of cellular functions such as inhibition of many secretions, cell proliferation and cell survival (Patel, 1999, Front Neuroendocrinol. 20:157-198). Somatostatins are broadly distributed in the centeral nervous system, peripheral nervous system, pancreas and gut (see, e.g., Watt et al., 2008, Mol Cell Endocrinol. 286: 251-261; Epelbaum, 1986, Prog. Neurobiol. 27: 63-100; and Raynor, 1992, Crit. Rev. Neurobiol. 6: 273-289). Somatostatins are also expressed in neuroendocrine tumors (NETs), such as medullary, thyroid cancer, neuroblastoma, ganglioneuroma, glucagonmas, adenocortical tumors and tumors that appear in the lung, paraganglia, duodenum and some other non-NETs (Volante et al., 2008, Mol. Cell. Endocrinol. 286: 219-229). Somatostatins can elicit effects on target cells by directly activating somatostatin receptors (SSTRs)(Watt et al., 2008, Mol Cell Endocrinol. 286: 251-261; Pyronnet et al., 2008, Mol. Cell. Endocrinol. 286: 230-237).
Somatostatin receptors (SSTRs) belong to a superfamily of G protein-coupled receptors (GPCRs) that each contain a single polypeptide chain consisting of extracellular/intracellular domains, and seven transmembrane domains. SSTRs are highly expressed in various cultured tumor cells and primary tumor tissues, including NETs (lung, GI, pancreatic, pituitary, medullary cancers, prostate, pancreatic lungcarcinoids, osteosarcoma, etc.) as well as non-NETs (breast, lung, colarectal, ovarian, cervial cancers, etc.) (Reubi., 2003, Endocr. Rev. 24: 389-427; Volante et al., 2008, Mol. Cell. Endocrinol. 286: 219-229; and Schulz et al., 2003, Gynecol. Oncol. 89: 385-390). To date, five SSTR receptor subtypes have been identified (Patel et al., 1997, Trends Endocrinol. Metab. 8: 398-405). SSTR2 in particular is expressed at a high concentration on many tumor cells (Volante et al., 2008, Mol. Cell. Endocrinol. 286: 219-229; and Reubi et al., 2003, Eur. J. Nucl. Med. Mol. Imaging 30: 781-793), thus making it a candidate target antigen for bispecific antibody cancer target therapeutics. In view of the high concentration of SSTR2 expressed on various tumors, it is believed that anti-SSTR2 antibodies are useful, for example, for localizing anti-tumor therapeutics (e.g., chemotherapeutic agents and T cells) to such SSTR2 expressing tumors. For example, bispecific antibodies to SSTR2 and CD3 that are capable of localizing CD3+ effector T cells to SSTR2 expressing tumors are believed to be useful cancer therapeutics. While bispecifics generated from antibody fragments suffer biophysical and pharmacokinetic hurdles, a drawback of those built with full length antibody-like formats is that they engage co-target antigens multivalently in the absence of the primary target antigen, leading to nonspecific activation and potentially toxicity. The present invention solves this problem by introducing novel bispecific antibodies directed to SSTR2 and CD3.